A monster modifier is one of the additional special attributes a monster can have, such as being extra strong or fast. Monsters change between the difficulties, and bosses spawn with all sorts of interesting properties.

Hit points

Minions are the monsters that spawn around a boss. They share some properties with the boss (such as Extra Fast and Extra Strong), and are tougher than normal monsters. They also receive considerable bonuses to their hit points, compared to standard monsters of their type.

Normal = 3x hit points.

Nightmare = 1.75x hit points.

Hell = 1.5x hit points.

Champions receive hit point bonuses over standard monsters of their type.

Normal = x3 hit points.

Nightmare = 2.5x hit points.

Hell = 2x hit points.

Uniques have hit point bonuses over standard monsters of their type.

Normal = x4 hit points.

Nightmare = 3x hit points.

Hell = 2x hit points.

Level

The level of monsters is factored into the determination of what type and quality of items they can drop, as well as their odds to be hit and to hit characters.

Champions = +2 levels

Uniques (bosses) = +3 levels

Experience

Champions and randomly-spawning bosses are worth more experience than standard monsters of their type. These bonuses apply to a boss' minions as well.

Champion: 3x

Unique: 5x

Champion Monster Bonuses

A variety of different types of Champions were added in D2X. They receive various bonuses to make them tougher than standard monsters of their type. Most champion monsters spawn in bright colors, making them stand out from standard monsters. Ghostly champions are translucent, and are the only type of Hell Bovine that look any different from all the rest.

Champions

Damage x2

+33-50% Elemental Damage

Attack Rating +75%

Attack Rate +120%

Velocity x2

+20% Speed

Ghostly

Half Speed

About 20% Chance for Cold Damage

80% Physical Resistance

Note: This modifier will set the physical resistance to 80% not increase it by 80%. You may found a "Immune to Physical" ghostly monster but it in fact only has 80% resistance and the description is wrong.

Fanatics

+100% Speed

-70% To Attack Rating

Berserkers

Damage x4

Attack Rating x4

Hit Points x1.5

Possessed

Hit Points x12

Can't be Cursed

Unique Monster Bonuses

Boss Properties are semi-randomly generated. Random bosses spawn with 1 random property on normal, 2 on Nightmare, and 3 on Hell. Superuniques generally have one or more fixed properties at all times, and many of them gain 1 additional on Nightmare and 2 on Hell. Act Bosses and Uber monsters do not get random properties; all their properties are preset, and they all grow more difficult on higher difficulties.

Cursed

If Unique hits, 50% of the time, an Amplify Damage Curse of (monster level / 4) is cast on attacker.

If any of a character's minons are Cursed, the curse will automatically spread to the character and all his/her minions.

Cursed can be inflicted by other monster properties, such as Auras that affect enemies (like Holy Freeze), or the Frost Nova explosion Cold Enchanted bosses emit upon death.

Cold Enchanted:

Cold Minimum Damage: +66% damage

Cold Maximum Damage: +100% damage

Cold Length: +20

Chance to Hit: x2

Cold Resist: 75% (stacks on top of existing monster resistance)

Cast Frost Nova of Monster Level upon death.

In Nightmare and Hell, Minions gain +33-50% Cold Damage.

Extra Fast

The boss and his minions get a big bonus to movement speed.

Attack Rate: x2 (Boss only; not minions.)

Fire Enchanted

Fire Minimum Damage: +66% damage

Fire Maximum Damage: +100% damage

Chance to Hit: x2

Fire Resist: 75% (stacks on top of existing resistance)

Explodes upon death, dealing 75%-100% of max hit points over a 4 yard radius. (This death explosion was nerfed in v1.09. Prior to that version, a number of very high hit point Superuniques on Hell difficulty could instantly kill any Characters in range.

In Nightmare and Hell, Minions gain +33-50% Fire Damage.

Lightning Enchanted

Lightning Minimum Damage: +66% damage

Lightning Maximum Damage: +100% damage

Chance to Hit: x2

Lightning Resist: 75%

If Monster is hit, a Monster level Charged Bolt is released. (This property was insanely dangerous when combined with Multi Shot, until v1.10 fixed a bug that causes the bolts to stack excessively.

In Nightmare and Hell, Minions gain +33-50% Lightning Damage.

Magic Resistant

Fire Resistance: 40%

Lightning Resistance: 40%

Cold Resistance: 40%

Mana Burn

Mana Minimum Damage: +(Minimum Damage x 4)

Mana Maximum Damage: +(Maximum Damage x 4)

Chance to Hit: x2

Magic Resistance: +20%

Multishot

Two or more missiles of the same type are shot. Most types of monsters gain additional shots on Hell.

Only spawns on monsters with ranged attacks.

Until v1.10 this property malfunctioned when combined with Lightning Enchanted, and sent out massive stacks of bolts, capable of instantly killing many characters.

Spectral Hit

Spectral hit adds elemental damage to the monster's attack, and adds 20% resistance to all those elements, but only if those resistances are less than 75%.

Fire Resist: +20%

Lightning Resistance: +20%

Cold Resistance: +20%

Chance to Hit: x2

One randomly chosen: Fire, Lightning, Magic, Cold, or Poison:

Element Minimum Hit: +66% damage

Element Maximum Hit: +100% damage

In Nightmare and Hell, the boss' minions gain +33-50% damage to one randomly selected element.

Stone Skin

Increases a monster's physical damage resistance. This property will often make monsters physically immune on Hell difficulty.

Damage Resist: +50%

Defense: x2

Extra Strong

A very dangerous property on tough melee fighters, this one increases the damage for the boss and his whole minion pack.

Minimum Damage: x2.5

Maximum Damage: x2.5

Chance to Hit: +25%

Minions do +75% damage.

Teleport

When attacked from melee range, bosses with this property will sometimes teleport to a random location when their hit points drop below 33%. The monster will partially heal when it teleports.

This property was very difficult to deal with in early versions of Diablo II, since it let monsters regain their full hit points each time they teleported. Battles between low damage characters and high hit point teleporting monsters could go on almost indefinitely. This property was nerfed in subsequent patches and in v1.09+ monsters teleport much less often, and heal only slightly when they do.

Some monsters, such as Baal and Uber Izual, will teleport without regaining hit points.

Thief

This property was removed from the game in v1.04. Before then it was most often noted as one of Lord de Seis' modifiers, as you can see in old screenshots. The thief property created a chance for potions to drop from the player's belt whenever the thief boss hit them with a melee or ranged attack. It was an interesting effect, but buggy, and could cause the game to crash under some circumstances. For instance, when a Barbarian is in mid-Whirlwind, his belt gets red'ed out, and he can not access his belt.